in this study, the cytogenetic effects of black tea and green tea were determined in cultured peripheral blood lymphocytes. results showed that black tea and green tea induced the mitotic and replication indexes and decreased micronuclei. but these data were not statistically significant for green tea. the effects of black tea on the micronucleus formation and mitotic index were statistically significant. the decrease in micronucleus counts indicated that black tea and green tea had considerable anticlastogenic and antigenotoxic effects as observed in vitro in human lymphocytes. thus, it could be concluded that tea polyphenols protected the normal cells from genotoxic or carcinogenic agents, which indicated the therapeutic and antioxidative role of catechins, flavonoids or other tea compounds.